1. Field of the Invention
The present invention relates to a servo control method and servo control apparatus in a feed drive mechanism configured to drive one mobile object by means of a plurality of motors.
2. Description of the Related Art
A method of controlling a position and velocity or torque of an object to be moved by means of a plurality of motors is known (ex. Jpn. Pat. Appln. KOKAI Publication No. 7-110714). An example of a feed drive mechanism configured to drive one object to be moved by using a plurality of motors is shown in FIG. 5. This mechanism is configured to drive one ball screw 34 by means of motors 21 and 22 arranged at both ends thereof to move a saddle 36 along guides 30 and 31. The ball screw 34 is supported by bearings 33 and 38, and a nut 35 is fixed to the saddle 36.
A block configuration diagram showing a conventional control system configured to control the feed drive mechanism is shown in FIG. 6.
In this control system, although the two motors are provided, the position/velocity control system is constituted of one system, and position/velocity control is carried out by position feedback and velocity feedback by using an encoder 23 of the motor 21.
The encoder 23 outputs a position signal for electrical angle calculation in accordance with the rotation of the motor 21. This position signal is output as a binary numerical value indicating a rotor rotation angle of the motor 21 obtained by dividing one revolution into, for example, segments of 1/220. Likewise, an encoder 24 also outputs a position signal for electrical angle calculation. As the motors 21 and 22, for example, synchronous motors are used. The motor is not limited to the synchronous motor, and a DC motor can be used in accordance with the use.
A position feedback producer 26 produces (calculates) a drive object movement amount per position control period. That is, the position feedback producer 26 produces mechanical position feedback obtained by dividing an amount of change in an output value from the encoder 23 per position control period by a number of counts per revolution of the encoder, multiplying the above obtained value by a movement amount of the mechanism (object to be driven) per revolution of the encoder, and then integrating the multiplication results.
A subtracter 11 subtracts the mechanical position feedback from a position instruction transmitted from a main control section (not shown) to output a position error. A multiplier 12 multiplies the position error by a position control gain Kp, and outputs the multiplication result as a velocity instruction.
A differentiator 13 differentiates a position signal output from the encoder 23, and outputs the differentiation result as a velocity feedback signal. A subtracter 15 subtracts a value of the velocity feedback signal from a value of the velocity instruction, and outputs the subtraction result as a velocity error. A velocity control processing section 18 subjects the velocity error to velocity control such as proportional integral operation or the like, and outputs the control result as a torque instruction. It should be noted that differentiation of the control system of the digital sampling system can be replaced with difference calculation.
A current control amplifier 19 converts the position signal from the encoder into an electrical angle, generates a motor drive signal on the basis of the electrical angle and torque instruction, and rotationally drives the motor 21. A control amplifier 20 also operates in a manner identical with the control amplifier 19.
A transfer function from the torque of such a feed drive mechanism to the velocity, i.e., a frequency characteristics of the velocity feedback signal which is the output of the differentiator 13 for the torque instruction which is the output signal of the velocity control processing section 18, obtained by simulation is shown in FIG. 7.
FIG. 7A shows a transfer function of a case where the saddle 36 is positioned at an end close to the motor 21, FIG. 7B shows a transfer function of a case where the saddle 36 is positioned at a part in the vicinity of the center, and FIG. 7C shows a transfer function of a case where the saddle 36 is positioned at an end close to the motor 22. As can be seen from FIGS. 7A to 7C, the transfer function changes depending on the position of the saddle 36 and, in the case of FIG. 7C where the saddle 36 is positioned close to the motor 22, a part in which the phase abruptly lags at a relatively low frequency occurs as indicated by an ellipse 25. In general, when the phase lag is 180°, and gain is greater than 0 db, the operation becomes unstable, and control becomes difficult to carry out. Accordingly, the characteristics of the control system shown in FIG. 7C become a hindrance to enhancement of the gain of the velocity control system, thereby causing a problem that it becomes difficult to sufficiently increase the gain. That the gain cannot be sufficiently increased implies that a sufficient response speed cannot be obtained.